Recently, a tire pressure monitoring system (TPMS), which detects a decrease in air pressure of a tire mounted in a vehicle, and notifies a driver of the decrease in air pressure, has been mounted in the vehicle.
When air pressure of a tire is low, there is a possibility that a vehicle easily slides, which may cause a big accident, the amount of fuel consumption is increased, so that fuel efficiency deteriorates, a lifespan of a tire becomes short, and riding comfort and braking power are considerably decreased.
The TPMS notifies a driver of a drop of pressure of a tire, and makes the driver check a pressure state of the tire, thereby preventing the aforementioned problem.
The TPMS is divided into a direct scheme TPMS for directly measuring a decrease in pressure of a tire, and an indirect scheme TPMS for indirectly measuring a decrease in pressure of a tire by using a wheel speed.
In the direct scheme TPMS, a pressure sensor attached to a tire notifies a driver of a change in air pressure of the tire. The indirect scheme TPMS indirectly estimates a change in air pressure of a tire through a change in a response characteristic (for example, a rotation speed, or a frequency characteristic of a rotation speed) of the tire generated when air pressure is decreased, and notifies a driver of a result of the estimation.
The indirect scheme TPMS may be classified into a dynamic loaded radius (DLR) analysis scheme, and a resonance frequency method (RFM) analysis scheme. The DLR analysis scheme and the RFM analysis scheme will be abbreviated as a radius analysis and a frequency analysis, respectively.
The frequency analysis adopts a scheme of, when air pressure of a tire is decreased, detecting a difference between a tire with normal air pressure and a tire with the decreased air pressure by using a change in a frequency characteristic of a rotation speed signal of a wheel. In the frequency analysis, taking notice of a resonant frequency obtainable by the frequency analysis of the rotation speed signal of the wheel, when the resonant frequency is calculated to be lower than a reference of frequency estimated during initialization, it is determined that the air pressure of the tire is decreased.
The radius analysis adopts a scheme of detecting a decrease in air pressure by comparing rotation speeds of four tires by using a phenomenon in which a dynamic loaded radius of a tire with decreased air pressure is decreased during travelling, and as a result, the tire with the decreased air pressure rotates faster than a tire with normal air pressure (see Japanese Patent Application Laid-Open No. 1988-305011).
However, the direct scheme TPMS has a problem in that an additional component for measuring pressure of a tire is further required, so that a configuration becomes complex, and manufacturing costs increase. Accordingly, the indirect scheme TPMS is used.
Particularly, the indirect scheme TPMS mainly estimates air pressure of a tire based on a wheel sensor signal of an anti-lock braking system (ABS) or an electronic controlled suspension (ECS).
The wheel sensor of the ABS or the ECS generates a speed signal by using a tone wheel and a hall sensor. The tone wheel is a mechanical structure generated by a sintering method, and generally has about 47 to 49 teeth while rotating one round.
Referring to FIG. 1, an ideal angle between the teeth may be calculated as 2 π/N. Here, N indicates the total number of teeth.
However, in actual, in a measurement angle between the teeth, an angle error is added to the ideal angle due to a processing error and the like. That is, the angle error may be calculated by a difference between the measurement angle and the ideal angle.
However, even though the angle error is included, an angle for one rotation of the tone wheel is constant with 360°, and the sum of the respective angle errors for one rotation is 0.
However, as illustrated in FIG. 2, when an measurement error, or disturbance, such as mechanical impact, is applied, when an angle is measured, the sum of the angle errors is not 0. In this case, when a frequency is estimated, it is impossible to accurately estimate a frequency by disturbance.
Accordingly, since a value measured by the wheel sensor is not accurate, there is a problem in that t is difficult to accurately estimate air pressure of a tire.